Digital images are communicated by values that represent the luminance and chromatic attributes of an image at an array of locations throughout the image. Each value is represented by a given number of bits. When bandwidth, storage and display requirements are not restrictive, sufficient bits are available that the image can be displayed with virtually uninhibited visual clarity and realistic color reproduction. However, when bit-depth is restricted, the gradations between adjacent luminance or color levels can become perceptible and even annoying to a human observer. This effect is apparent in contouring artifacts visible in images with low bit-depth. Contour lines appear in low frequency areas with slowly varying luminance where pixel values are forced to one side or the other of a coarse gradation step.
These contouring artifacts can be “broken up” by adding noise or other dither patterns to the image, generally before quantization or other bit-depth reduction. This noise or pattern addition forces a random, pseudo-random or other variation in pixel values that reduces the occurrence and visibility of contours. Typically, the image is perceived as more natural and pleasing to a human observer.
Some of these methods can be explained with reference to FIG. 1, which illustrates an image display system 1. In these systems, noise or dither patterns 16 can be added to 4 or otherwise combined with an image 2. The combined image is then quantized 6 to a lower bit-depth. The image may then be displayed directly or, as shown in FIG. 1, may be transmitted 8 to a receiver 10. After reception, the noise/dither 16 that was added to the image may be subtracted 12 or otherwise de-combined with the image to reduce the visible effect of the noise/dither on areas where contouring is not likely to occur. The image is then displayed 14 on the receiving end. These methods may also be used in systems that do not transmit or receive such as with displays with bit-depth capabilities that are lower than the image data 2 to be displayed.
Some of these methods may be explained with reference to FIG. 2. In these systems 20, an image 2 is combined 28 with a noise/dither pattern 16 and sent to a display system 22 that cannot display the full range of image data contained in the image. These display systems 22 may quantize 24 the image data to a bit-depth that matches the display capabilities. The quantized image data is then displayed on the display 26.
In the systems illustrated in FIG. 2, the noise/dither pattern is not subtracted or de-combined from the image. In these systems, less noise can be added to an image before it causes adverse visual impact or “graininess.” Various frequency distributions for noise/dither patterns have been found to be more or less visible to the human visual system. Generally, the human visual system works as a low-pass filter that filters out high frequency data. Therefore, noise concentrated in a high-frequency range is less visible than lower frequency noise.
Often it is not feasible to use a dither/noise pattern that is as big as an image file. In these cases, a smaller dither pattern can be used by repeating the pattern across the image in rows and columns. This process is often referred to as tiling. In multiple image sets, such as the frames or fields of video images, a dither pattern may be repeated from frame to frame as well. Dither patterns may be designed to minimize artifacts created by their repetitive patterns.
Dither structures may comprise multiple dither patterns to be used across a single image of multiple frames. A three-dimensional dither structure, as shown in FIG. 3, may employ a series of dither patterns. These patterns 30-36 may be arranged in a sequence that is used on sequential frames of video. A first dither pattern tile 30 may be used on a first video frame 38 while a next sequential pattern 32 is used on a next successive video frame 40. The sequence of patterns 30-36 may be repeated after each pattern in the sequence is used. These sequences may also be specially designed to reduce the occurrence of artifacts from their repetitive temporal patterns.